1. Field of Invention
Embodiments of this invention relate to materials for electro-optic devices, electro-optic devices that use the materials and methods of producing the materials and electro-optic devices. More particularly, embodiments of this invention relate to materials for electro-optic devices that are blends of a metal oxide and at least one other material that increases electrical conductivity and/or decreases a work function of a layer of such a material when blended with the metal oxide, to electro-optic devices that use the materials and methods of producing the materials and electro-optic devices.
2. Discussion of Related Art
The contents of all references referred to herein, including articles, published patent applications and patents are hereby incorporated by reference.
Electronic devices based on organic materials (small molecules and polymers) have attracted broad interest. Such devices include organic light emitting devices (OLEDs) (Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913), organic photovoltaic cells (OPVs) (Tang, C. W. Appl. Phys. Lett. 1986, 48, 183), transistors (Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Appl. Phys. Lett. 1996, 69, 3066), bistable devices and memory devices (Ma, L. P.; Liu, J.; Yang, Y. Appl. Phys. Lett. 2002, 80, 2997), etc. Some of the most salient attributes of polymer electronics is that they can be very low-cost, flexible, operate with low-energy consumption, can be produced with high-throughput processing, and can be versatile for applications (Forrest, S. R. Nature 2004, 428, 911). To fulfill the requirement of low cost, a solution process is highly desirable. Within the field of organic electronics, the metal/organic interface plays a critical role in determining the device performance. The interface can often be modified by some functional interfacial layer in order to improve device performance. Depending on the characteristics of the material, the functional interfacial layer can be employed in different configurations. Early prominent examples in the development of OLEDs and OPVs include—(1) Introducing LiF, CsF, AlOx, etc. as an electron buffer layer in OLEDs (Hung, L. S.; Tang, C. W.; Mason, M. G. Appl. Phys. Lett. 1997, 70, 152; Grozea, D.; Turak, A.; Feng, X. D.; Lu, Z. H.; Johnson, D.; Wood, R. Appl. Phys. Lett. 2002, 81, 3173; Kin, Z.; Hino, Y.; Kajii, H.; Ohmori, Y. Mol. Cryst. Liq. Cryst., 2007, 462, 225); (2) Application of polyaniline (PANI) (Yang, Y.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 1245) and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport/buffer layer (Jonas, F.; Krafft, W.; Muys, B. Macromol. Symp. 1995, 100, 169); (3) Insertion of a TiOx layer as an optical spacer/hole blocking layer (Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.; Heeger, A. J. Adv. Mater. 2006, 18, 572; Hayakawa, A.; Yoshikawa, O.; Fujieda, T.; Uehara, K.; Yoshikawa, S. Appl. Phys. Lett. 2007, 90, 163517); (4) Combination of an n- and p-type transport layer for tandem OLEDs (e.g. LiF-V2O5) (Chu, C.-W.; Chen, C.-W.; Li, S.-H.; Wu, E. H.-E.; Yang, Y. Appl. Phys. Lett. 2005, 86, 253503).
Solar cells, also known as photovoltaic (PV) cells or devices, generate electrical power from incident light. The term “light” is used broadly herein to refer to electromagnetic radiation which may include visible, ultraviolet and infrared light. Traditionally, PV cells have been constructed of a number of inorganic semiconductors, e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. More recently, PV cells have been constructed using organic materials.
Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs as well as other possible advantageous properties.
PV devices produce a photo-generated voltage when they are connected across a load and are irradiated by light. When irradiated without any external electronic load, a PV device generates its maximum possible voltage, V open-circuit, or VOC. If a PV device is irradiated with its electrical contacts shorted, a maximum short-circuit current, or ISC, is produced. (Current is conventionally referred to as “I” or “J”.) When actually used to generate power, a PV device is connected to a finite resistive load in which the power output is given by the product of the current and voltage, I×V. The maximum total power generated by a PV device is inherently incapable of exceeding the product ISC×VOC. When the load value is optimized for maximum power extraction, the current and voltage have values, Imax and Vmax, respectively. A figure of merit for solar cells is the fill factor, ff (or FF), defined as:
  ff  =                    I        max            ⁢              V        max                            I        SC            ⁢              V        OC            where ff is always less than 1, as ISC and VOC are never achieved simultaneously in actual use. Nonetheless, as ff approaches 1, the device is more efficient.
When electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, for example, an organic molecular crystal (OMC) material, or a polymer, a photon can be absorbed to produce an excited molecular state. This energy absorption is associated with the promotion of an electron from a bound state in the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), or equivalently, the promotion of a hole from the LUMO to the HOMO. In organic thin-film photoconductors, the generated excited state is believed to be an exciton, i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. The excitons can have an appreciable life-time before recombination. To produce a photocurrent the electron-hole pair must become separated, for example at a donor-acceptor interface between two dissimilar contacting organic thin films. The interface of these two materials is called a photovoltaic heterojunction. If the charges do not separate, they can recombine with each other (known as quenching) either radiatively, by the emission of light of a lower energy than the incident light, or non-radiatively, by the production of heat. Either of these outcomes is undesirable in a PV device. In traditional semiconductor theory, materials for forming PV heterojunctions have been denoted as generally being of either n (donor) type or p (acceptor) type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states. The type of the background majority carrier concentration depends primarily on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the HOMO-LUMO gap. The Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to ½. A Fermi energy near the LUMO energy indicates that electrons are the predominant carrier. A Fermi energy near the HOMO energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the PV heterojunction has traditionally been the p-n interface.
A significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. As opposed to free carrier concentrations, carrier mobility is determined in large part by intrinsic properties of the organic material such as crystal symmetry and periodicity. Appropriate symmetry and periodicity can produce higher quantum wavefunction overlap of HOMO levels producing higher hole mobility, or similarly, higher overlap of LUMO levels to produce higher electron mobility. Moreover, the donor or acceptor nature of an organic semiconductor may be at odds with the higher carrier mobility. The result is that device configuration predictions from donor/acceptor criteria may not be borne out by actual device performance. Due to these electronic properties of organic materials, the nomenclature of “hole-transporting-layer” (HTL) or “electron-transporting-layer” (ETL) is often used rather than designating them as “p-type” or “acceptor-type” and “n-type” or “donor-type”. In this designation scheme, an ETL will be preferentially electron conducting and an HTL will be preferentially hole transporting.
Organic PV cells have many potential advantages when compared to traditional silicon-based devices. Organic PV cells are light weight, economical in the materials used, and can be deposited on low cost substrates, such as flexible plastic foils. However, organic PV devices typically have relatively low quantum yield (the ratio of photons absorbed to carrier pairs generated, or electromagnetic radiation to electricity conversion efficiency), being on the order of 1% or less. This is, in part, thought to be due to the second order nature of the intrinsic photoconductive process. That is, carrier generation requires exciton generation, diffusion and ionization. However, the diffusion length (LD) of an exciton is typically much less than the optical absorption length, requiring a trade off between using a thick, and therefore resistive, cell with multiple or highly folded interfaces, or a thin cell with a low optical absorption efficiency.
For polymer solar cells, the polymer/fullerene based bulk-heterojunction (BHJ) solar cell is the most common device architecture (Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864; Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.; Ree, M. Nat. Mater. 2006, 5, 197; Sievers, W. D.; Shrotriya, V.; Yang, Y. J. Appl. Phys. 2006, 100, 114509) for which a recently certified solar cell efficiency of 5.4% in a single cell configuration was achieved. Recently we showed that when salts like Cs2CO3 or CsF are applied as an n-type interfacial layer between the polymer active layer and the aluminum electrode, photovoltage (open-circuit voltage Voc), fill-factor (FF) and device efficiency all improve. In addition, application of Cs2CO3 (both by thermal evaporation and solution processes) as an effective electron injection/buffer layer leads to record high white and red PLEDs with significantly reduced driving voltage and enhanced lifetime (Huang, J.; Xu, Z.; Yang, Y. Adv. Funct. Mater. 2007, 17, 1966). Combined with novel p-type interfacial layers such as transition metal oxides (V2O5, MoO3, WO3 etc.), we successfully demonstrated efficient inverted polymer solar cells (Li, G.; Chu, C.-W.; Shrotriya, V.; Huang, J.; Yang, Y. Appl. Phys. Lett. 2006, 88, 253503; Shrotriya, V.; Li, G.; Yao, Y.; Chu, C.-W.; Yang, Y. Appl. Phys. Lett. 2006, 88, 073508). Furthermore, the sol-gel process titanium sub-oxide (TiOx) and zinc oxide (ZnO) were recently shown to be an effective n-type buffer material for improving solar cell efficiency (Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864). However, there remains a need for improved organic photovoltaic devices.